1. Field of the Invention
The present invention relates to an organic electroluminescence element (heretofore referred to as an organic EL element). In particular, the present invention relates to a display panel which uses an organic EL element where at least one organic layer is formed by a relief printing method.
2. Description of the Related Art
An organic EL element emits light by applying electrical current to a luminescent layer made of an organic luminescence material which is formed between two electrodes facing each other. The film thickness of a luminescent layer is important so that an organic EL element may emit light efficiently. Therefore, the film is required to be a thin film with a thickness of about 100 nm. In addition, patterning with a high definition is necessary in order to use an organic EL element as a display in which colors are separated by luminescent colors, such as, R (red), G (green) and B (blue).
As an organic luminescence material which forms a luminescent layer, a low-molecular material and a high-molecular material can be used. In general, a thin film of a low-molecular material is formed by a resistance heating evaporation method or the like, and in this case, patterning is performed by using a mask which has a fine pattern. However, there was a problem that the larger a substrate became, the more difficult it became to obtain patterning accuracy.
Then, recently, a method for forming a thin film by a wet coating method with a coating liquid which is made by dissolving a high-molecular material in a solvent has been practiced. As a wet coating method of forming a thin film, a spin coating method, a bar coat method, an extrusion coat method and a dip coat method are examples. However, it is difficult to pattern with a high definition and form three colors, that is, R, G and B, separately by these wet coating methods. Therefore, it is thought that a formation of a thin film by a printing method which is superior in application of different colors and patterning is the most efficient. Actually, as a practice by a printing method, a method performed by an offset printing method (Patent Document 1) and a method performed by a relief printing method (Patent Document 2) have been proposed.
As a basic structure of a high-molecular organic EL element which uses a high-molecular material as an organic luminescence material, a structure in which a luminescent medium layer such as a hole transport layer, an interlayer, a luminescent layer or an electron injection layer are stacked respectively between an anode and a cathode is used generally. Among the above, a layer having an organic compound is called an organic layer in the present invention.
An interlayer which is formed as a buffer layer between a hole transport layer and a luminescent layer in this luminescent medium layer is expected to have an effect of enhancing hole transport properties and an effect of blocking an electron coming from a cathode side. Actually, the fact that efficiency and lifetime of an organic EL element are improved by forming an interlayer has been confirmed.
A high-molecular material which is used for an interlayer can be dissolved in an organic solvent and can be made into a coating liquid the same as a material which forms a luminescent layer. Therefore, the thin film of the interlayer can be formed by a wet coating method. In addition, a display panel can be manufactured by a method of applying an interlayer to an entire surface of a substrate in one go such as a spin coating method, a bar coat method, an extrusion coat method and a dip coat method, because a separate application for three colors of RGB is not required for forming an interlayer, though it is required for forming a luminescent layer.
However, in an actual substrate of a panel for a display, there are some parts which have a problem, when an organic layer is applied. For example, a connection wiring portion which is for connecting to an electrode and a part which is for connecting to a driver for driving. Therefore, when an organic layer is applied to an entire surface of a substrate, it is necessary to remove an unnecessary part of the organic layer.
Moreover, since a luminescent layer is formed on the interlayer, a condition of a film formation of a luminescent layer changes depending on a condition of a film formation of the interlayer. In other words, if a film of an interlayer is formed nonuniformly, a film of a luminescent layer is formed nonuniformly due to unevenness such as surface asperity and a difference in wetting properties. Therefore, emission irregularity occurs.
Thus, a purpose of the present invention is to provide an organic EL display panel and a method of manufacturing an organic EL display panel in which emission irregularity is decreased by improving evenness of a film thickness of particularly an interlayer which is one of the organic layers formed by a wet coating method.    Patent Document 1: JP-A-2001-93668    Patent Document 2: JP-A-2001-155858